1. Field of the Invention
The present invention relates to piston units, and more particularly piston units having rolling membranes.
2. Description of the Prior Art
Piston units, such as pumps, valves and pressure-compensating devices are used frequently in the handling of different types of pumpable media. In cases of high demands on tightness, washability and hygiene, such as for example in the handling of pumpable foodstuffs, the piston unit often is provided with a rolling membrane, that is to say a flexible membrane which is connected in a liquid-tight manner to the cylinder wall and is adapted to be in contact with the piston and form a tight barrier between it and the pumped medium. For the handling of foodstuffs, and in particular wholly or partly sterilized foodstuffs, a second membrane situated at the opposite end (piston rod end) of the piston is frequently also used, the space between the two membranes being connected to a source of vacuum. As a result the space situated between the membranes will serve as a barrier between the pumped goods and the environment, a possible leak being indicated immediately owing to its effect on the vacuum.
In piston units of the abovementioned type provided with rolling membrane, the required low pressure is created with the help of an external device, e.g. a vacuum pump or the like, which via a line is connected to the space between the two rolling membranes. The line opens into the cylinder wall on a level with the piston which means that during the movement of the piston one or both rolling membranes from time to time will cover the opening of the vacuum duct, which is a disadvantage, since during this time the vacuum cannot be acted upon or controlled which entails the danger of a possible leakage not being immediately detected.
To make sure that the rolling membrane situated at the free surface of the piston, i.e. the one not provided around a piston rod, makes contact completely and symmetrically with the piston end without the formation of folds or bubbles, it is essential that the vacuum prevailing in the space between the two rolling membranes is distributed completely evenly. This is especially difficult to secure in the case of pistons of a relatively large diameter and plane end face, since the rolling membrane after contact with the peripheral edge of the piston surface prevents the vacuum from reaching the volume enclosed between the piston top and the rolling membrane. An enclosed air volume is therefore produced between the end face of the piston and the membrane which entails the formation of folds in the membrane and appreciably enhances the risk of asymmetrical stressing of the membrane, which during prolonged operation may lead to crack formation and leakages which are disastrous in the hygienic handling of previously sterilized foodstuff products.